Communication using current detection has been used in current loop communications technology for many years. For example, in the 1960's, military teleprinters used 60 milliamp current loops to communicate. In later periods, 20 milliamp current loops became more prevalent. In a conventional current loop, a current source provides a specified current on a loop. A signal is transmitted on the current loop by opening and shutting a switch within the loop. A receiver utilizes a current detector to receive the transmitted signal. Current loops have the advantage that they allow longer communications paths than certain other technologies. Converters may be used to convert signals to, for example, RS-232 format for communication with computers.
FIG. 1 illustrates a full-duplex current loop. In this instance, both parties to the communication can send and receive a signal simultaneously. In this illustration, first participant 100 and second participant 110 communicate with each other. First participant 100 in FIG. 1 is an “active” participant because first participant 100 provides the current sources 140 and 170 for the current loop system. Second participant 110 is a “passive” participant. In such a conventional full-duplex current loop, it is possible for either or both participants to be active, depending on the design of the system.
The system shown in FIG. 1 includes two circuit loops, a first circuit loop 120 and a second circuit loop 130. In first circuit loop 120, first participant 100 is the receiving participant and second participant 110 is the transmitting participant. In second circuit loop 130, first participant 100 is the transmitting participant and second participant 110 is the receiving participant.
Within first circuit loop 120, current source 140 produces a current through the circuit loop. Second participant 110 transmits a signal on first circuit loop 120 by opening or closing switch 150. First participant 100 then receives the signal transmitted by second participant 110 by detecting the current in first loop 110 using current detector 160. Similarly, within second circuit loop 130, current source 170 produces a current through the circuit loop. First participant 100 transmits a signal on second circuit loop 130 by opening or closing switch 180. Second participant 110 then receives the signal transmitted by first participant 100 by detecting the current in first loop 130 using current detector 190.
FIG. 2 illustrates a simplex, or half-duplex, current loop. In this instance, only one party to the communication can send a message at any time. In this illustration, first participant 200 and second participant 210 communicate with each other. First participant 200 in FIG. 2 is the “active” participant because first participant 200 provides the current source 230 for the current loop system. Second participant 210 is a “passive” participant. In a conventional simplex current loop, it is possible for either participant to be the active participant depending on the design of the system. The system shown in FIG. 2 includes a single circuit loop 220. In the circuit loop 220, first participant 200 and second participant 210 alternate as the transmitting participant and the receiving participant.
Within circuit loop 220, current source 230 produces a current through circuit loop 220. In one instance, first participant 200 transmits a signal on circuit loop 220 by opening or closing switch 240. Second participant 210 then receives the signal transmitted by first participant 200 by detecting the current in circuit loop 220 using current detector 270. In a second instance, second participant 210 transmits a signal on circuit loop 220 by opening or closing switch 250. First participant 200 then receives the signal transmitted by second participant 210 by detecting the current in circuit loop 220 using current detector 260.
The conventional current loops illustrated in FIGS. 1 and 2 have characteristics that limit the usefulness of these communications systems. Among other issues, the conventional transmission of a signal by opening and closing a circuit connection creates an unbalanced transmission signal that results in significant noise problems. While communications over relatively long distances are possible with a current loop system, the noise that is present on a conventional current loop system limits the speed that can be realized by the system, and thereby limits the usefulness of the system in modern high-speed communications.